(chief sources: Internet sites of U.S. government weather and emergency agencies and American Academy for the Advancement of Science)

Electrons and light

An atom is composed of a positively-charged nucleus that has anywhere from one to a few dozen negatively-charged electrons orbiting it. (Picture your head surrounded by a swarm of mosquitoes.) An atom is electrically stable when the number of electrons is just right to balance the number of positive charges within the nucleus. When an atom loses one electron or more, it becomes a positively-charged ion – more positive charges than negative charges. (A mosquito has left and there’s room for another one to join the swarm.) When an atom gains, rather than loses, one or more electrons, it becomes a negatively-charged ion.

Ions with either imbalance are usually somewhat to very unstable and will attempt to balance the electrons as quickly as possible. A molecule, (think of a cluster of atoms sharing a swarm of electrons), behaves electrically the same way as an individual atom. A molecule may also lose or gain electrons and become an unstable ion. The electrons around a nucleus at any one moment are in orbits – some close to the nucleus and some sort of in layers of orbits at increasing distances from the nucleus. Electrons remain rather reliably in these orbits, until something upsets the balance of the atom or corrects the imbalance of the ion.

When an electron escapes one orbit and jumps to another on a single atom or molecule, or when an electron escapes a molecule and moves toward another, the energy that bound it in orbit is commonly emitted as light.

Put simply, common lightning is the illumination created by the energy given off when a surge of electrons flows in an immense, sudden burst from a negatively-charged mass of ions (a cloud) to a positively-charged mass of ions (the ground). The same illumination from a flow of electrons happens on a minute scale at the tip of a spark plug, a piezo element (gas grill igniter), or you may even have seen it when you remove a sweater or when you “zap” something or someone with static electricity.

The role of clouds
Ice particles, from tiny crystals to large hailstones, appear to be very important in creating huge electrically-charged areas within a cloud. Owing to the rising and sinking air associated with thunderstorms, these ice particles collide frequently inside the cloud. These collisions within the cloud, which can take place between crystals that collide with other crystals thousands of times within a few seconds – whether they move great distances or only tiny distances – can cause these particles to lose or gain electrons and thereby build up electrical charges. Without the forces of a storm to hold the charged particles tightly in place within a cloud, charged particles in the atmosphere find opposite charges and resolve their instability quietly.

Due to the different rates at which particles rise and fall within a storm cloud, concentrations of electrically-charged particles can be found. As the thunderstorm grows, intense electrical fields can develop within the cloud mass. A large positive charge forms in the frozen upper part of a cloud and two charged regions – a large, negatively charged region and a smaller positively charged region – form in the lower portion of the cloud.

The earth’s surface normally maintains a small negative charge with respect to the atmosphere, but when a thunderstorm drifts overhead, the overwhelming negative charge at the cloud base induces a positive charge on the ground below the storm. Another way to think of it is to remember the principle that like charges repel each other, so molecules in the ground compensate by giving up electrons and becoming positively charged ions. The positive ground charge becomes an electrical current that follows the movement of the cloud like a shadow and concentrates on elevated objects, such as trees, buildings, and higher portions of terrain, in an attempt to establish a current (attract electrons away from the cloud) in order to equalize the charges between the cloud base and the ground.

Air, however is a good insulator, so the electrical potential between cloud and ground must build up from tens of millions to hundreds of millions of volts before the insulating properties of the air break down and an ionized conductive channel is established for the current to flow between the two charges. If you have ever had your hair stand on end during a thunderstorm, (or as one approached), you were in this positive ground current, and could have become a lightning rod. The most common form of cloud-to-ground lightning results when negative ions become massed inside the portion of the cloud nearest the ground.

(“Heat lightning” generally refers to the discharge of electrons from a negatively-charged region to a positively-charged region within the storm’s cloud mass.)

Lightning is usually initiated within the thunderstorm cloud when a faint, negatively charged channel called the “stepped leader” emerges from the base of the cloud and propagates toward the ground in a series of steps of about 1 microsecond in duration and 30 meters in length. The stepped leader reaches from cloud base to ground in about a hundredth of a second. As the stepped leader approaches the ground, streamers of positive charge rush upward from objects on the ground. When one of the streamers from the ground contacts the leading edge of the stepped leader from the cloud, the lightning channel is opened, a negative charge starts flowing to the ground, and a return stroke, lasting about a tenth of a second, propagates through the channel as a bright luminous pulse. Sometimes, following the initial return stroke, one or more additional leaders may propagate down the decaying lightning channel at intervals of about a tenth of a second. These leaders, called dart leaders, are not stepped or branched like the original leader, but are more or less direct and continuous. Like the stepped leader, however, they initiate return strokes. These return strokes are what we call lightning.

Since a lightning bolt can be seen for many miles, we might think it is several meters wide, but actually the channel of electrons is only about a pencil-width. The glow from the released energy (refraction by air molecules) is what makes the flow of light energy seem wider. (A distant bare light bulb creates a glow out of proportion to its diameter, as does the filament within it.) Also the amount of current that a lightning strike packs is not as great as we might guess. An ordinary lightning strike would power an ordinary household light bulb for only a few months.

A lightning bolt, then, can be loosely described as a flow of electrons from the sky to the ground. But it would be simplistic to imagine that a river of electrons is ejected from a cloud and rushes to the ground. Electrons do not exist in gushing torrents; except when jumping from one molecule to the next, each electron must be attached to an atom or a molecule.

How do charges flow?
Electrons become “excited” when a positive charge is nearby. When a nearby positive charge is strong enough, an “excited” electron in the vicinity will separate from its orbit on one molecule and leap toward the positive charge. The energy released in the separation is expressed as light. As untold trillions of trillions of electrons make the jump in the direction of a positively-charged area almost simultaneously and in the same charged channel, their individual bursts of light translate into a brilliant lightning bolt. In the course of a lightning strike, as soon as an electron has jumped to a new molecule nearby and found an orbit, the attraction of the positively-charged ground makes that electron jump from its newfound orbit on one molecule, emit its burst of light, and join another molecule, and repeat the process until it has reached the ground or dissipated by settling onto a molecule outside the channel in which the lightning is flowing.

The excess electrons from the negatively-charged ions in the cloud, then, accomplish their transfer of charges by jumping to the next molecule in the air, then to the next molecule, and so on, on their way to the ground. This transfer of electrons follows the “leaders” extending from the cloud mass, and the process accelerates as the excited electrons get nearer to, and feel the increased pull from, the surge of positive charge rushing upward from the ground.

Reverse, or “positive” lightning
Not all lightning forms in the negatively charged area low in the thunderstorm cloud. Some lightning forms in the cirrus anvil at the top of the thunderstorm. This area carries a large positive charge, and lightning from this area can carry that positive charge to a negatively-charged area on the ground. This type of lightning stroke is particularly dangerous for several reasons. It frequently strikes away from the rain core, either ahead of or behind the thunderstorm. It can strike as far as 5 or 10 miles from the storm, in areas most people wouldn't consider risky for lightning. And positive lightning typically has a longer duration, which results in more electrical charge being transferred in the flow of electrons from the ground to the cloud. This can allow for easier ignition of fires and an increased risk to an individual who is struck.

When you consider that the ground directly under a storm cloud, which of course can be the size of a township or a whole county, has a concentration of positively-charged ions, it makes sense that a little beyond that area of positively-charged earth (several miles beyond, that is) will be an area with a concentration of negatively-charged ions, because the easily-attracted electrons in the ground around a storm have been induced into the positively-charged ground beneath the storm. As the storm is raging in one county and electrons are about to flow from the bottom of the cloud toward the earth, many miles away the negatively-charged ground can attract the attention of the positively-charged region in the anvil at the top of the cloud that is being ignored by the excess electrons in the base of the cloud. Thus a distant thunderstorm can be dangerous due to the much more rare “reverse lightning” just described.

What is thunder?
During a lightning strike, air along a lightning discharge channel is instantaneously heated to temperatures near 10,000 degrees Celsius. This happens for a couple of reasons, perhaps especially because the sudden movement of air molecules along a lighting channel causes friction, which creates heat. It involves such a mass of molecules and happens so rapidly that all of the molecular movement, apart from the transfer of electrons, is expressed as heat energy.

Air is comprised of many substances in their the gaseous state. The rapidly expanding gases in a lightning bolt push against the surrounding air, which cushions the expansion for a moment, but resists the expansion as a rubber balloon will resist pressure inside it. The lightning channel then rapidly cools, creating a vacuum. The rapid expansion is then reversed by the combination of the atmosphere’s resistance and the vacuum. When the lightning discharge channel slams shut, a shock wave is created, the shock wave that we hear as thunder. As with other loud sounds, the atmosphere can bounce echoes of the wave for several seconds.

When lightning strikes “right on top of you,” the sound it makes is a violent “crack.” You’re not likely to hear rolling thunder from a near hit, although it sounds like normal thunder to anyone at a distance.

How can you tell how far a lightning strike is from you?
Thunder can typically be heard as far as 10 to 15 miles away from a thunderstorm. Since light travels faster than sound, the further you are from a thunderstorm, the more time will pass after you see a lightning strike and before you hear the thunder.

Sound travels at about a thousand feet (two tenths of a mile) per second. To estimate your distance from a lightning strike, count the seconds. Five seconds will be about a mile. Occasionally a strike will appear to be close but you won’t hear the thunder. That doesn’t mean it wasn’t close, but that other atmospheric conditions momentarily absorbed or deflected the shock waves that created the thunder. And when you see several lightning bolts in a few seconds, you may not be able to tell which rumble of thunder goes with which lightning strike.

In a violent lightning storm, when you can see lightning striking every two to five seconds for instance, it can be impossible to determine the distance by counting five seconds per mile. Which late-arriving rumble went with which of the bursts of light that occurred over the course of the last few seconds?

What does lightning do to you if you’re hit?
Your nervous system is an electrical system. The lightning – (the electrons that are flowing from a negatively-charged sourced to a positively-charged source) – is trying to follow the path of least resistance in order to balance the charges between two masses of charged ions. When you are part of that path of least resistance, lightning travels along your nerve pathways. It may also travel across the surface of your skin, especially if you are wet. (Electricity doesn’t travel well through water, but water that is rich in dissolved minerals or ions – and perspiration would match that description– can make a good conductor.)

Major nerve pathways extend into the arms and legs, and if lightning travels these paths, a victim can be temporarily paralyzed and then recover. A victim may experience amnesia and recover. A person who is hit by lightning is conducting only a part of the flow of electrons, and not always enough to cause permanent damage. A person may suffer burns or other tissue damage at the points where the current enters and exits the body.

Injuries vary. If the current takes a turn and exits through the abdomen, the damage will be different from that which goes in one arm and out the other, or straight through a vertical path. A person’s muscles may spasm forcefully when struck by lightning, and the victim may be “thrown” through the air. This has been known to happen when a person inside a house touched the kitchen faucet at the wrong moment and was thrown across the room as lightning traveled through the house’s plumbing.

If the victim receives enough of the current to cause death, it is usually the result of the current’s ability to stop the heart by interfering with its natural beating, regulated by the nervous system.

What does lightning do when it hits an object?
On a solid electrical conductor, an electrical current – which is what happens when an electron is forced off its host molecule and bumps an electron off the next molecule and so on – follows the object’s outside layer of molecules. That’s why you’re ordinarily safe inside a car or an airplane. For decades, the Museum of Science in Boston has offered a demonstration of what happens when a person is inside a large steel cage at the moment the cage is struck by lighting, (created in the museum by a Van de Graaff generator). The path of least resistance will not be through the car or the cage, but around it. You’d be safe inside a metal building for the same reason. However in neither case should you touch anything metal that is connected to the outside of the enclosed space.

What is ball lightning?
Ball lightning is a rare form of lightning that really doesn't look like lightning at all. Instead, it's a glowing ball of light typically the size of a grapefruit, but sometimes as small as a pea or as large as a bus. It usually forms from the most violent thunderstorms. Sometimes moving very fast, but more often described as hovering or drifting feather-like, ball lightning can either roll along the ground or atop fences, can pass through open buildings, or can float thousands of feet in the air – sort of like electrically charged beach balls. They can either last several seconds, or several minutes. Usually, they don't cause any damage, but have been known to leave burn marks if they pass through windows or screens.

Those who’ve had the closest encounters with ball lightning describe various colors from blue to orange to white, an “unkempt” appearance as opposed to a polished, smooth sphere, sometimes an irregular hissing or sputtering, without odor, and, oddly, without heat – possibly even cold.

The behavior of ball lightning has caused many witnesses to avoid sharing their stories with others for fear of being ridiculed. But with the acknowledgment of science that ball lightning is an accepted, if unexplained, phenomenon, more accounts are being documented. While some accounts describe ball lightning that set a building afire or struck and even killed people, more common are the accounts of it entering a building or even a parked vehicle through glass or solid walls and then exiting. Even though not stopped by walls in some instances, in other cases it has apparently traveled the perimeter of a room or building as if not able to find an escape. It has been seen to reverse direction and leave the way it came. It has also been seen to stop and then explode, indoors or out.

But the most mysterious thing about ball lightning is, to this day, scientists can't explain what causes it. One hypothesis holds that, because lightning is based on forces of positive and negative charges that are subject to magnetism, strong magnetic fields created within a thunderstorm can compel bursts of excited electrons to behave in bizarre ways.

One witness has described what appeared to be a lightning bolt turn itself into a ball and then split into an ever greater number of smaller ones that dissipated into the storm. Another, related idea holds that the mass of charged ions forms a ball for much the same reason that a water droplet assumes the shape of a sphere; the particles have nowhere to call “home,” are held together by a force greater than that which would pull them apart, and forming a ball reduces the surface area to a minimum, which is the most efficient way of responding to the force that holds them together.

Another theory posits that, in the vacuum of a lightning strike, a portion of the electrons have become separated from the lightning bolt and have chanced to form a (hollow) ball or bubble during their rush toward a source of positive charges – the ground, for instance. Think of a glass blower making a sphere; that, like wood shavings curling away from the blade on a sharp knife or plane, some electrons are sheered off from the lightning bolt and find a fleeting stability in the roundness of the sphere. But being free electrons and still in search of positive ions, they remain excited and illuminating.* They have no mass, so they are not affected by gravity or wind. A few stray positive ions in the otherwise-insulating atmosphere are not sufficient to attract all the electrons at once, so none can leave the sphere until all can be absorbed in the same moment. But these are only theories, and testing them is exceedingly difficult.

*Although the laws of physics deny atomic particles the ability to exist separately from complete molecules, an untested and frankly untestable hypothesis suggests that ball lightning consists of the nebulous ethereal substance – no longer electrically charged – that formed when a mass of electrons (or other atomic particles) literally did escape the grip of molecules during a lightning strike. If electrons, which are something if not actual matter, could not find molecules to unite with during a lightning strike, what would become of them?

Lightning safety tips
=> If you hear thunder from a storm, you are in danger of being struck by lightning. Move indoors.
=> A vehicle offers good protection from lightning as long you do not touch any metal that is connected to the outside, such as a metal door handle or the metal trim around a window. => If you are caught in your vehicle during an electrical storm, stay there unless you are in danger for another reason, (a flash flood or tornado).
=> If your hair stands on end or you feel a tingling sensation in your skin, you are very close to being struck by lightning. Positive charges are trying to use your height to extend streamers skyward to meet a stepped leader from the clouds. Immediately crouch near the ground. Do not lie flat since that would increase the amount of electrical current your body will receive if lightning were to strike close to you.
=> Do not use a cord telephone unless it is an emergency. Lightning easily travels through phone lines.
=> Do not touch any plug-in appliances during a thunderstorm. Computers, televisions, stereos, air conditioner compressors, and refrigerators are vulnerable to relatively small power surges. Unplug them if possible, (but don’t wait until lightning is striking nearby).
=> Unless you have all plastic plumbing, do not take a shower or bath during a thunderstorm. Lightning travels easily along metal pipes.
=> As soon as you hear thunder, stop outdoor activities and take shelter.
=> If someone is struck and you are nearby, start first aid immediately, including CPR if necessary. A lightning victim does not retain an electrical charge, and most victims can be saved if treated immediately.